Frames for vehicles, especially towable recreational vehicle trailers and motorized recreational vehicles (RVs), are typically made of steel. Steel is certainly a durable material for producing a frame, but it does have disadvantages. Weight is a primary drawback to using steel for framing an RV. The weight in a steel frame is due to the considerable weight of the material itself and also due to its shape. Generally, RV frames are made from readily available I-beams or tubular sections of a standard size. These I-beams and tubes are of a constant cross section along their entire length. However, this does not place the most material in areas of high stress where it is needed most nor does a constant cross sectional beam reduce material where less is needed as in areas of low stress.
Assembling a steel vehicle frame is difficult. During assembly there will typically be two longitudinal beams running the length of the RV and stringers or cross members will be placed across the longitudinal beams. At each junction of the stringers and longitudinal beams a connection must be made. These connections are made by welding or mechanically fastening the beams and stringers together. This takes a great deal of time and provides potential for mistakes. After the beams and stringers are mated, an additional step of attaching a deck must be made so that an RV may be built upon the frame. The deck that is attached to the steel frame may be made of wood, aluminum, or could structural steel. Attaching the deck to the frame is accomplished with many screws driven through the deck into the frame. This method of joining the deck to the frame is not only time consuming, but does not produce a continuously integrated deck and frame. In the case that the deck is wood, it must be protected from the elements below.
The geometry of a steel frame for an RV is not ideal. As mentioned above, uniform cross sections have the same height at the ends and at the middle. Ideally, less material would be placed at the ends so that more adequate ground clearance could be had at the front and rear of the vehicle. Also, in attaching the stringers to a traditional longitudinal beam, height is added to the vehicle as the stringers rest on top of the beams. This is height that is not available for living space within the vehicle due to height restrictions. This added height can also raise the center of gravity of the vehicle, which may be detrimental to the handling of the vehicle.